Close portable recordings of the RockValley earthquake sequence con#rm the unusually shallow 2 km average hypocentral depths, and provide data for an investigation of the source parameters. Stress drops are estimated using both a spectral #tting technique that #rst corrects for attenuation, and a deconvolution technique that inherently accounts for attenuation. The shallow depths suggest a relatively low level of shear stress acting on the RockValley fault, and allow an estimation of seismic e#ciencies. The data allow the possibility of large stress drops, on the order of 100 bars, implying seismic e#ciencies much greater than 0.1. This has important implications for the unresolved issue of the strength of faults in general. A dependence of stress drop with seismic moment remains unresolvable with this data. However, the possibility of partial stress drops and non-linear responses does exist. A seismic survey designed speci#cally for the purpose of measuring attenuation could resolve t..